Ultimate Marvel

Part re-imagining, part Adaptation Distillation, the Ultimate line was originally an attempt by Marvel Comics to tempt readers into comics without being scared off by the decades of Backstory and Retcons in the official universe.This attempt was launched in November, 2000. This had already been attempted by various other experiments - the Marvel Comics 2 comic line, The New Universe, Heroes Reborn, etc., but none of those met with the success the Ultimate Marvel line did, except for Spider-Girl (the original MC2 title, and via multiple relaunches the only survivor of the line), which became the longest running Spidey spin-off ever.

As basically an experiment which does not affect the main universe, changes and revelations are more permanent, and the universe as a whole has plot lines and continuity more neatly intertwined; notably, the connections to the Weapon X project from Ultimate X-Men and the history of the Super SoldierSerum/CaptainAmerica from The Ultimates. As the line has grown and accumulated its own continuity, its focus has shifted from distilling old stories to making all kinds of sweeping changes, like killing X-Men's sacred cows or replacing Peter Parker as Spider-Man.

The Ultimate Marvel universe is part of Marvel's multiverse, specifically Earth-1610, while the main Marvel Earth is Earth-616 (though former EIC Joe Quesada despises the designation). Marvel has insisted doing a crossover between the two would be a lame and tired idea. At one point, it seemed that Ultimate Fantastic Four would have such a crossover, but it turned out to be a trick by denizens of a strange Alternate Universe: the one of the Marvel Zombies in their debut appearance. A true crossover has been announced for 2012, teaming up the mainstream and Ultimate Spider-Men, though with Ultimate Peter Parker dead by that point the circumstances are very different than when the original statement was made.

Note that technically, the vol. 2-and-later titles have been renamed from "Ultimate _____" to "Ultimate Comics: _____"; we're just sticking with the original format here for convenience (face it, titles like "Ultimate Comics: New Ultimates" are redundant and kind of a mouthful).

Ultimate Marvel Team-Up, a series where Spider-Man teamed up with various Ultimate heroes; it was more episodic and light than the main Ultimate Spider-Man series. At least partly Canon Discontinuity, some elements are referenced by later series but other elements are contradicted.

Ultimate Daredevil and Elektra and Ultimate Elektra: Devil's Due.

Ultimate Adventures, an odd book about a Batman parody that lost a competition to get upgraded to an ongoing. Not officially Canon Discontinuity, but don't expect anyone to mention it outside of Official Handbooks to Ultimate Marvel.

Ultimate War, an X-Men/Ultimates crossover. Magneto shows up after being thought dead, thanks to the fact that Xavier mindwiped him instead of killing him. The government ain't happy, and they send out the Ultimates.

Ultimate Six, a Spider-Man/Ultimates crossover. Spidey's Rogues Gallery escapes imprisonment (forming an Ultimate take on the Sinister Six) and they go after him with the Ultimates in pursuit.

The Ultimate Galactus trilogy, consisting of Ultimate Nightmare, Ultimate Secret, and Ultimate Extinction. As you might guess from its name, the Eater of Worlds is headed towards Earth and the Ultimates, X-Men and Fantastic Four have to band together to drive it off.

Ultimate Vision, a miniseries spun off from Ultimate Galactus.

Ultimate X4, an X-Men/Fantastic Four crossover. Someone steals Cerebro from the X-Men and frames the Four for it.

Ultimate Iron Man, two miniseries detailing the character's origin. Canon Discontinuity thanks to Mark Millar, who later established that this was a fictional in-universe TV show so that he could go in other directions with Tony's past.

Ultimate Power, a crossover between Ultimate Marvel and Supreme Power.

Ultimate Wolverine vs. Hulk, which got out two issues and then went on hiatus for four years.

Ultimate Human, billed as Iron Man versus The Hulk, and derailed somewhat by the appearance of the Ultimate Leader.

Ultimate Armor Wars, an adaptation of the classic Iron Man story, it features Tony Stark trying to recover an item and discovers that someone's stolen his Iron Man tech and sold it to various criminals that now employ it.

The Ultimate Enemy trilogy, a Spider-Man/Fantastic Four crossover consisting of Ultimate Enemy, Ultimate Mystery, and Ultimate Doom. Superhumans and scientific installations have been targeted by a new mystery Big Bad.

Ultimate Fallout: Spider-Man No More: As the world mourns Peter Parker, the superhero community starts considering and planning for the future...

Vol. 3:

Ultimate Hawkeye: Tying in to The Ultimates; Nick Fury sends Hawkeye, the Hulk, Karen Grant, Firestar and Guardian to investigate and retrieve a super-powering 'Serum' developed by the S.E.A.R. (South-East Asian Republic) military.

Breaking the Fellowship: Happened to the X-Men and Fantastic Four in Ultimatum due to the losses each suffered in that event. The remaining X-Men eventually regrouped, especially after Spider-Man died, but the Four were cemented as broken up forever when Reed turned evil as a Knight Templar.

Genetic Engineering Is the New Nuke: Stated as such by Nick Fury at the end of Ultimate Six. Outside of the Asgardians, just about every existing superhuman can be traced back to military experiments in some way.